1. Technical Field of the Invention
The present invention relates generally to a method of adjusting an output of a gas sensor element which may be employed in air-fuel ratio control for an internal combustion engine of automotive vehicles.
2. Background Art
A typical gas sensor element employed in the air-fuel ratio control for automotive vehicles includes a solid electrolyte body made of an oxygen ion conductive material, a gas measuring and a reference gas-exposed electrode, and a diffused resistance layer. The diffused resistance layer is disposed on a surface of the target gas-exposed electrode exposed to a target gas to be measured. The target gas, thus, reaches the target gas-exposed electrode through the diffused resistance layer.
When the voltage is applied to the target gas-exposed electrode and the reference gas-exposed electrode, the current flowing through these electrodes is determined as a function of the number of oxygen molecules passing through the diffused resistance layer. The current flowing through the electrodes, thus, shows characteristics that it is saturated at a given value as long as the concentration of oxygen in a target gas is constant.
FIG. 16 represents the relation between the voltage applied to the target gas-exposed electrode and the reference gas-exposed electrode and the current output picked up from the electrodes for difference concentrations a to d of oxygen (a greater than b greater than c greater than d). The drawing shows that application of a suitable voltage, for example, voltage V to the target gas-exposed electrode and the reference gas-exposed electrode causes the current to flow through the electrodes as a function of the concentration of oxygen. For instance, when the concentration of oxygen is a, the current Ia flows through the electrodes. This is the principle of measurement of the concentration of oxygen in the above gas sensor element.
However, when the above type of gas sensor elements are mass-produced, they may have a unit-to-unit variation in the above described characteristics. If there is no unit-to-unit variation, the application of voltage to the target gas-exposed electrode and the reference gas-exposed electrode of each gas sensor element exposed to a target gas whose concentration of oxygen is a will cause the current Ia to be, as shown in FIG. 16, produced by the electrodes. If, however, there is the unit-to-unit variation, the currents produced by the gas sensor elements show the distribution, as shown in FIG. 17. Some of the gas sensor elements producing the currents outside the range xcex94Ia will produce great measurement errors that are objectionable in practical use.
In order to eliminate the unit-to-unit variation of the gas sensor elements caused by production errors, Japanese Utility Model Second Publication No. 7-27391 teaches use of a correction circuit which corrects the output current of each gas sensor element. This method, however, results in complexity of the whole circuit structure of gas sensors and an increase in manufacturing cost.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a simple and low-cost adjusting method of adjusting an output of a gas sensor.
According to one aspect of the invention, there is provided a gas sensor output adjusting method of adjusting a sensor output of a gas sensor element. The gas sensor element includes a lamination of a solid electrolyte body, a target gas-exposed electrode, a reference gas-exposed electrode, and a diffused resistance layer in which a target gas to be measured diffuses. The target gas-exposed electrode is disposed on a first surface of the solid electrolyte body exposed to the target gas. The reference gas-exposed electrode is disposed on a second surface of the solid electrolyte body exposed to a reference gas. The diffused resistance layer is disposed on the first surface of the solid electrolyte body. The target gas-exposed electrode and the reference gas-exposed electrode produce the sensor output. The adjustment of the sensor output is achieved by decreasing a diffusion length of the target gas in the diffused resistance layer as a function of a quantity of the sensor output to be adjusted.
In the preferred mode of the invention, the decreasing the diffusion length is achieved by removing a portion of the diffused resistance layer.
The diffused resistance layer includes a porous layer and a dense layer. The decreasing the diffusion length may be achieved by removing a portion of the porous layer.
The diffused resistance layer may include only the porous layer.
The decreasing the diffusion length may alternatively be achieved by removing a portion of the dense layer so as to broaden an area of the porous layer exposed to the target gas.
According to the second aspect of the invention, there is provided a gas sensor output adjusting method of adjusting a sensor output of a gas sensor element. The gas sensor element includes a lamination of a solid electrolyte body, a target gas-exposed electrode, a reference gas-exposed electrode, and a diffused resistance layer in which a target gas to be measured diffuses. The target gas-exposed electrode is disposed on a first surface of the solid electrolyte body exposed to the target gas. The reference gas-exposed electrode is disposed on a second surface of the solid electrolyte body exposed to a reference gas. The diffused resistance layer is disposed on the first surface of the solid electrolyte body. The target gas-exposed electrode and the reference gas-exposed electrode produce the sensor output. The adjustment of the sensor output is achieved by decreasing a gas-diffusing sectional area of the diffused resistance layer within which the target gas diffuses as a function of a quantity of the sensor output to be adjusted.
In the preferred mode of the invention, the diffused resistance layer includes a porous layer and a dense layer. The decreasing the gas-diffusing sectional area of the diffused resistance layer is achieved by partially sealing a surface of the porous layer exposed to the target gas.
The decreasing the gas-diffusing sectional area of the diffused resistance layer may alternatively be achieved by forming a plurality of output-adjusting holes in the dense layer which lead to the porous layer and sealing a given number of the output-adjusting holes as a function of the quantity of the sensor output to be adjusted.
According to the third aspect of the invention, there is provided a gas sensor output adjusting method of adjusting a sensor output of a gas sensor element. The gas sensor element includes a lamination of a solid electrolyte body, a target gas-exposed electrode, a reference gas-exposed electrode, and a diffused resistance layer in which a target gas to be measured diffuses. The target gas-exposed electrode is disposed on a first surface of the solid electrolyte body exposed to the target gas. The reference gas-exposed electrode is disposed on a second surface of the solid electrolyte body exposed to a reference gas. The diffused resistance layer having an outer surface exposed to the target gas, an inner surface opposite the outer surface, disposed on the first surface of the solid electrolyte body, and side surfaces formed between the outer and inner surfaces, defining portions of side surfaces of the lamination. The target gas-exposed electrode and the reference gas-exposed electrode produce the sensor output. The adjustment of the sensor output is achieved by decreasing a diffusion length of the target gas in the diffused resistance layer as a function of a quantity of the sensor output to be adjusted by removing a portion of the diffused resistance layer obliquely to at least one of the side surfaces of the lamination.